Control of Blood Vessels: Peripheral Resistance Flashcards
What does the blood vessel radius depend on
active tension exerted by smooth muscle (vascular smooth muscle)
passive elastic properties of wall (elastin and collagen)
blood pressure inside vessel
(tissue pressure outside vessel - veins)
What is the law of laplace?
distending pressure = wall tension / radius
where pressure = intra - extravascular pressure
What maintains vessel calibre?
distending pressure and Tw (wall tension)
Rearrange equation for distending pressure to find wall tension and radius
Tw = P x r
r = Tw / P
Compare wall tension between small and large vessels and why?
smaller in small vessels
increase in radius increases wall tension
What may occur if distending pressure is too high?
vessel rupture
>1000mmHg
How do blood vessels resist increasing pressure?
elastic tissue develops tension passively in response to a rise in pressure to confer stability
adish
What can smooth muscle develop?
active tension, which is independent of distending pressure
What does active control of vessel calibre allow?
redistribution of blood flow
control of pre/post capillary sphincters
regulation of vascular tone and control of blood pressure
What is vascular tone?
degree of constriction/dilatation
In vasoconstriction what tension is acting?
increased active tension
decreased passive tension
In vasodilatation, what tension is acting?
decreased active tension
increased passive tension
Which vessels does vasomotor tone describe?
arterioles and arteries
Which vessels does venomotor tone describe?
venules and vein
What factors affect vascular smooth muscle contraction?
nerves hormones endothelium-derived vasorelaxants endothelium-derived vasoconstrictors metabolites myogenic mechanisms
What other NTs may cause vasocontriction?
ATP and NPY
tend to work with NA
How do nerves affect vascular smooth muscle?
noradrenaline released from sympathetic nerve varicosity
binds to α2 and α1 on SM - vasoconstriction
binds to β2 - vasodilation
primarily activates α1 adrenoceptors in most vascular beds
How do hormones affect vascular smooth muscle?
Catecholamines -
eg. noradrenaline, adrenaline (constrict/dilate)
Peptides - eg. vasopressin, angiotensin (constrict)
bradykinin (dilate)
Which endothelium-derived vasorelaxants affect vascular smooth muscle?
PGI2 - prostacyclin
NO - nitric oxide
EDHF - endothelium-derived hyperpolarising factor
Which endothelium-derived vasoconstrictants affect vascular smooth muscle?
endothelin
How do metabolites affect vascular smooth muscle?
increase lactate, increase H+, released K+, generation of adenosine from breakdown of ATP - vasodilatation
systemic circulation - high O2, low CO2 - localised constriction
pulmonary vasculature - high O2, low CO2 - dilatation
VICE VERSA
Where is myogenic mechanisms important?
important in cerebral vasculature
and kidney
Why are myogenic mechanisms important?
important contributer to autoregulation of blood flow
How do myogenic mechanisms affect vascular smooth muscle? (autoregulation of blood flow)
increased pressure is sensed, arteries stretch causing vasoconstriction
offsets increased flow (due to increased pressure) by increasing resistance
What are extrinsic control mechanisms primarily concerned with?
regulation of arterial BP
How does this autoregulatory range help predict pathologies?
if increase in pressure exceeds certain value, vessels cannot contract enough - risk of stroke
pressure decreases too much, too much dilatation - risk of unconsciousness and ischaemic damage
What differs between different vascular beds?
function differs can behave differentially depending on the situation
